Let me ask you, then: who should this woman have cosplayed, that would have fit her body type? Off the top of my head, I can't think of any major characters, male or female, that match her body shape. The reasons for this are pretty obviously tied to our cultural conceptions of beauty—women should be slender and curvy. There's obviously some exceptions, but most are still of a pretty average build.

Should she be disallowed from the hobby, or ridiculed for doing something she enjoys, just for this?

Additionally, I know a fair number of cosplayers choose the characters they do because they identify with the character's struggles and characteristics, and not solely because they look cool.

Caitlin describes Lara Croft as "feminine but dangerous, well-educated but athletic". By dressing as her, becoming her, she's taking on some of those traits. If it boosts her morale, if it makes her feel better about herself, because she is sees herself in that way, or is acting in that manner, why should she let her weight and build be a barrier to positive mental health?